Overlooked vinyl gold mines

Your local library! That's right. Many municipal libraries have annual or semi-annual book sales. Most material sold at these events come from donations by library patrons. My library always has an enormous amount of vinyl given to them by people cleaning out basements and attics. Just recently, I bought a box full of Soviet Union recorded and pressed LP's on the Melodya lable in EX+ to NM condition. All are Russian orchestras, performing Russian music. Some of the performances are great, some are average, but, the sonics are great. The price? .25/LP Keep your eyes on your local paper, ask the staff when the next sale is going to be. Better yet, ask them if they have taken in any records, get to know them, and when the sale starts, you will be ready. You get great deals, and your money goes to help support programs at your local library.
The library system in my area actually has had to open up their very own used record store (within their used book store) in a local mall to handle all the stock coming through. They used to to sell all the records right at the libraries, but they've cut that back now. It can be a little tough to shop at this place, since there's a ton of records, many of which have been around a long time without selling (and for good reason), with not much organization or knowledgeable sorting-out and catagorizing going on, but by the same token, if you're willing to slog through a lot of the same dead wood repeatedly, you can still find good recent arrivals over the course of a patient afternoon. It's not my most productive spot, but it's pretty impressive nonetheless, not to mention cheap.
I have been stopping by libraries on trips that I take around the country. Its hit and miss but generally pretty good hunting quality wise and they do not price by the piece so rare stuff is much cheaper than dealers. The other place I go is thrift shops. Quality is lower, but I generally find a really nice one for a dollar out of every hundred dogs.